The present invention relates to a shift control system for a manually shiftable automatic transmission and, more particularly, to a shift control system for an automatic transmission capable of switching a manual shift mode for selecting a gear stage on the basis of a manual operation and an automatic shift mode for selecting a gear stage on the basis of a running state.
In the automatic transmission of the prior art, the power generated by an engine is transmitted through a fluid coupling to a transmission mechanism for achieving one of gear stages. For this achievement, the transmission mechanism is equipped with a plurality of planetary gear units which are composed of elements including ring gears, carriers and sun gears, so that the elements may be selectively engaged/disengaged by frictional engagement means to achieve the gear stages.
For engaging/disengaging the frictional engagement means, there is further provided an oil pressure circuit for pumping a pressure-regulated working fluid to and from a hydraulic servo mechanism corresponding to each of the frictional engagement means, so that the frictional engagement means may be engaged/disengaged at a predetermined timing.
Moreover, the automatic transmission of the prior art is provided with individual ranges including P (parking), R (reverse), N (neutral), D (drive), S (second) and L (low) ranges, one of which is selected according to the intention of a driver to effect a shift automatically.
In an automatic transmission having four forward stages and one reverse stage, for example, an automatic shift is effected among the first to fourth speeds in accordance with the running conditions such as the vehicle speed and the throttle opening if the driver selects the D-range. If the S-range is selected, the shift is automatically effected among the first to third speeds. If the L-range is selected, the shift is automatically effected between the first and second speeds. The selection of each range according to the intention of the driver is carried out by moving the shift lever to the corresponding range position arrayed on a straight line. In other words, the range position can be selected from the "I"-shaped array.
Incidentally, the automatic transmission described above does not allow the gear stages to be freely selected in the manual manner so that it fails to give a good shifting feel. Thus, there has been proposed an automatic transmission in which the shift lever is turned in a shift passage to come into contact with a sensor so that a gear stage may be achieved in response to the electric signal of the sensor (as disclosed in Japanese Patent Laid-Open No. 8545/1990 or 125174/1990). In another proposal (as disclosed in Japanese Patent Laid-Open No. 157855/1986), there is provided a shift pattern which has the so-called "H-pattern" shift position like the manual shift in addition to the "I-pattern" range position used in the existing automatic transmission so that the automatic transmission can select a run in an automatic shift mode and a run in a manual shift mode.
The existing automatic transmission described above can provide a shifting feel because the driver can achieve a predetermined gear stage by manipulating the shift lever. However, the automatic transmission employs the same transmission mechanism, the same oil pressure circuit for actuating the transmission mechanism and the same control system for controlling the oil pressure circuit as those of the prior art and is constructed to prevent highly frequent shifts. Thus, the shift is not started before a predetermined time period elapses from the manipulation of the shift lever by the driver. This delays the shift response even in the manual shift mode so that the shifting feel is deteriorated.